Technical Field
The present disclosure relates to an integrated gas sensor device. The disclosure also relates to a manufacturing process of an integrated gas sensor device.
Description of the Related Art
As it is well known, a gas sensor is a device which detects the presence of various gases within an area, usually as part of a safety system. This type of equipment is used to detect a gas leak and interfaces with a control system so that a process can be automatically shut down. A gas sensor can also sound an alarm to operators in the area where the leak is occurring, giving them the opportunity to leave the area. This type of device is important because there are many gases that can be harmful to organic life, such as humans or animals.
In this field, a carbon monoxide sensor or CO sensor is a device that detects the presence of the carbon monoxide (CO) gas in order to prevent carbon monoxide poisoning. In fact, it is known that CO attaches to the hemoglobin (in the blood stream) with an affinity 200× stronger than oxygen, producing inadequate amounts of oxygen travelling through the body, with symptoms like headache, fatigue, nausea, and dizziness that could resemble many common illnesses, and thus being misdiagnosed and mistreated.
Elevated levels of CO can be thus dangerous to humans depending on the amount present and length of exposure. In particular, also small concentrations of CO can be harmful over longer periods of time while increasing concentrations are dangerous even in case of reduced exposure times.
The CO sensors are particularly important due to the fact that CO is colorless, tasteless and odorless (unlike smoke from a fire), and thus detection in a home environment is impossible without a suitable warning device.
At present carbon monoxide sensing devices are readily available for many industrial applications. The sensors used in these devices include electrochemical sensors, semiconductor sensors, colorimetric detectors and infrared detectors.
In the field of electrochemical sensors, the amperometric sensors are largely used to detect a wide range of electroactive gases, for instance carbon monoxide.
Amperometric gas sensors usually comprise three electrodes: a working electrode, a counter electrode and a reference electrode. These electrodes are placed in contact with an electrolytic medium, such as an electrolyte solution, or a polymer electrolyte. A potential is then applied to the working electrode in order to induce an electrochemical reaction of the sensed gas which generates a current, proportional to the gas concentration.
By selecting proper metal electrodes, electrolytes and working potentials, this same device structure may be made optimized for detection of different gaseous analytes (e.g., NOx; O2, CO2 . . . ).
The gas has to diffuse from the environment through a diffusion barrier and then reaches the working electrode surface. The working electrode is a so called gas diffusion electrode.
As for CO sensors, the electrochemical detection of carbon monoxide is based on the reaction of the same at the working electrode, usually made of platinum, which is able to oxidize carbon monoxide. Essentially, carbon monoxide is oxidized at the platinum working electrode to carbon dioxide while oxygen is consumed at the counter electrode. As previously indicated, the current that is generated is a measure for the amount of carbon monoxide.
Because of the chemical reaction of CO, the working and counter electrodes do not maintain a constant potential. Therefore a separate reference electrode is also necessary. All three electrodes are gas diffusion electrodes.
In particular, the working and counter electrodes are in contact with an external environment, and thus with the environmental air, via a gas diffusion barrier. The reference electrode is shielded from carbon monoxide and is kept in a constant environment and therefore maintains a constant potential. For the counter reaction, sufficient supply of oxygen is also important.
In case of a carbon monoxide detection, the electrochemical sensors have advantages over sensors obtained by other technologies in that they have a highly accurate and linear output to carbon monoxide concentration, utilize minimal power since operating at room temperature, and have a long lifetime. Main drawbacks of this kind of sensors are high prices and the need for frequent recalibration of the same.
Other problems are encountered when the gas sensors are to be miniaturized, the final price of the miniaturized sensor devices being even higher.
Known gas sensors are described for instance in the article to Maseeh et al. entitled “A Novel Silicon Micro Amperometric Gas Sensor”, IEEE, 1991, 91CH2817-5 and in the article to P. D. van del Wal et al. entitled: “The development of a Nafion based amperometric carbon monoxide sensor for domestic safety”, Analusis, 1999, 27, No. 4, EDP Sciences, Wiley-VCH.